How are the spins of two entangled photons measured at light speed?

and how is it known that the two photons are entangled in the first place? I mean before measuring how do you know that you have the correct two photons?

Walrus said:

and how is it known that the two photons are entangled in the first place? I mean before measuring how do you know that you have the correct two photons?

Aspect’s experiment deals with two entangled photons. He used calcium atom. It might be of your interest.

Walrus said:

How are the spins of two entangled photons measured at light speed?

With entangled photons we are working with polarization, not spin. Often measurement is done with a two-channel polarizer that deflects horizontally polarized photons in one direction and vertically polarized ones in another. Less sophisticated experiments can use simple polarizing filters.

and how is it known that the two photons are entangled in the first place? I mean before measuring how do you know that you have the correct two photons?

We need a reliable source of entangled photons; these days we use a procedure called “spontaneous parametrized downconversion” (Google if you are curious, but the details aren’t as important as the result) to produce them. If two photons show up at the right places at the same time chances are very good that they are a pair created by our pair source. Of course every once in a while two stray photons wandering through our lab will just happen to luck into our photon detectors at the same time; we collect our data across thousands of pairs so that one or two strays don’t significantly affect the results.

Nugatory said:

With entangled photons we are working with polarization, not spin. Often measurement is done with a two-channel polarizer that deflects horizontally polarized photons in one direction and vertically polarized ones in another. Less sophisticated experiments can use simple polarizing filters.We need a reliable source of entangled photons; these days we use a procedure called “spontaneous parametrized downconversion” (Google if you are curious, but the details aren’t as important as the result) to produce them. If two photons show up at the right places at the same time chances are very good that they are a pair created by our pair source. Of course every once in a while two stray photons wandering through our lab will just happen to luck into our photon detectors at the same time; we collect our data across thousands of pairs so that one or two strays don’t significantly affect the results.

And why doesn't measuring the first photon end the entanglement before the entanglement can begin? I mean there can be no distance traveled without a starting point and measuring p1 collapses it. Also why aren't there two distances traveled since the first photon is also traveling at light speed I find the people telling me to look it up, because they cannot find a link their selves humorous.

Walrus said:

And why doesn't measuring the first photon end the entanglement before the entanglement can begin?

Start with a bit of math:
A quantum state is represented by things called “kets” (Google for “bra-ket notation“) that look something like ##|HV\rangle## - this particular ket should be read as “That quantum state in which if we measure the polarization of the photon that reaches the left-hand detector we will find that it is horizontally polarized and if we measure the polarization of the photon that reaches the right-hand detector we will find that it is vertically polarized”.
An entangled pair will have a quantum state that looks like ##\frac{1}{\sqrt{2}}(|HV\rangle+|VH\rangle)## which should be read as “a 50-50 superposition of the states ##|HV\rangle## and ##|VH\rangle##”. This is a superposition, so a measurement will cause the state to collapse to one of the possible alternatives. Say we measure the particle at the left-hand detector and find it to be vertically polarized. We’ve just collapsed the initial wave function down to ##|VH\rangle##; this is the state in which we can be sure that if we measure the polarization of the particle at the right-hand detector it will be horizontally polarized. This collapse also breaks the entanglement because ##|VH\rangle## isn’t an entangled state; we can further manipulate the photon at one detector to turn that state into something like ##|Vx\rangle## (here we did something with the particle at the right-hand detector) but that doesn’t affect the measurement at the other detector.

So to answer your question: the entanglement began when the pair was created. They remained entangled until the first measurement; this measurement collapsed the entangled state into an unentangled one.

Be aware that I have cut many corners and committed some horrible oversimplifications in this answer. Unfortunately there’s no way of truly understanding this stuff without the math, and nothing short of a serious college-level textbook (minimum two years of multivariable calculus, differential equations, complex analysis, and linear algebra required) will do the math justice. However, Giancarlo Ghirardi has written a more layman-friendly book “Sneaking a look at God’s cards” that covers a lot of this ground - you might want to give it a try.

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